Breathing rate/heart beat rate/body movement are typical vital signs parameters used in clinical monitoring of patients. Heart and lung diseases typically affect breathing rate/heart beat rate. It may also be desired to know the body movement of bed ridden patients within a specific period of time. Therefore, monitoring of these vital signs parameters can be a significant diagnostic method in planning of medical care.
Electrical sensors have typically been used for monitoring of the parameters. For example, US 2007/0008156 describes using piezoelectric sensors. However, it has been recognized that electrical sensors are not sensitive enough to distinguish between, for example, shallow breathing and no breathing. In addition, electrical sensors are typically prone to electromagnetic interference (EMI), which can be a significant problem in certain clinical examinations, e.g., during magnetic resonance imaging (MRI) examinations.
As an alternative, use of optical fiber sensors has been explored. Optical fiber sensors are inherently immune from electromagnetic interference and are chemically inert. In “Optical fibre sensors embedded into medical textiles for heathcare monitoring”, IEEE Sensor J. 8 (7), 1215-1222, 2008, Grillet et al used a macrobending sensor. The sensor is a single mode fiber in a belt form for measuring respiratory rate. Although the interrogation is relatively simple and requires only low cost and compact components, a macrobending sensor typically has low sensitivity such that even if such a sensor is embedded within a bed, it is typically difficult to detect chest wall movement during breathing. The differences between macrobending and microbending in optical fibers is established. Macrobending typically causes light to leak out of a fiber due to macroscopic deviations of the fiber's axis from a straight fine. On the other hand, microbending is typically due to mechanical stress on a fiber that introduces local discontinuities which can result in light leaking from the core of the fiber to a cladding via mode coupling. In U.S. Pat. No. 6,498,652, although an optical phase interferometry-based fibre optic sensor was found to have higher sensitivity, the sensor system used is complex, leading to cost increases that may inhibit practical implementation. Grillet et al also tested fibre Bragg grating (FBG)-based sensors and optical time-domain reflectometer (OTDR)-based sensors for breathing rate measurement. However, it has been found that both such sensing systems are too complex and expensive.
In “A smart bed for non-intrusive monitoring of patient physiological factors”, Meas. Sci. Technol. 15, 1614-1620, 2004, Spillman et al proposed using a fibre optic statistical mode (STM) sensor and a high order mode excitation (HOME) sensor for breathing rate/heart beat measurements. However, it has been found that these sensors require highly coherent light sources and a bulky high order mode generator. In “Application of long period grating sensors to respiratory function monitoring”, Proc. SPIE, 5588, 148-156, 2004, Allsop et al used a long period grating-based sensor to monitor breathing rate. However, it has been found that the system is too complex and expensive. In “Monitoring of the Heartbeat Sounds using an Optical Fiber Bragg Grating Sensor”, IEEE Sensor conference, pp 306-309, 2005, Gurkan et al proposed using a FBG sensor for heart beat measurement. It has been found that although a FBG sensor-based system has relatively good sensitivity, the system cost is significantly high.
Therefore, there exists a need for a vital signs detecting device and a method for detecting vital signs that seek to address at least one of the problems.